MadSci Network: Engineering

Re: How do I build a Audio Driven Switch

Date: Mon Nov 15 01:23:58 1999
Posted By: Abtin Spantman, , Electrical Engineering, L. S. Research, Inc.
Area of science: Engineering
ID: 941763257.Eg

Hi Leo:

In concept, we need to build a 'Low-Pass filter'.  This filter allows the 
low frequency audio tones to get through, but stops the higher frequencies 
to get through.
There are many ways to build filters. The general catagorization may fall 
into two catagories of 'passive filters' and 'active filters'. A simple 
capacitor placed between the line and ground would be an example of a 
passive filter.  It lets high frequencies pass through it to ground, 
effectively killing the high frequencies, and letting only the low 
frequency or DC components to remain on the line.  I chose an active filter 
design as it does the above, but much better. 
I made some assumptions as follows.
1) I assumed the audio tone will be from an 'audio out' source, such 
as a tape recorder output, which is somewhere around 1Vpp. 
2) I assumed that such things as phase and amplitude distortion are really 
not an issue here.
3) I assumed that the light may be larger than a  flashlight type bulb.
4) I assumed that this is just to experiment with, to have fun, at a party 
perhaps lighting up on deep base tones. Furthermore, that it will not be 
used in any real application. 
Having said all that, here is the circuit with ideal values calculated:

The circuit pictured here had the following response when I simulated it 
using PSPICE:

But here is the catch - in real life, we can't get values accurate enough 
with decimal points.  Heck, sometimes we are lucky if we can get close at 
all.  So I went to the parts room, and I picked out the closest values that 
are commercially available, and proceeded to build the circuit shown below:

The simulation with the actual values matched up with what I actually 
observed in the lab. My lab prototype stopped at about 144Hz. Not knowing 
what you intend to use this for, I would venture to guess that it will 
probably do just fine for your application.  If, however, you need more 
accuracy, try to get as close to the ideal values as you can.
Here is the response you will see if you use the commercially available 

I added a transistor to drive a relay (labeled L2- ignore the 10uH) in the 
schematic. The relay can be used to drive a light of your choice. The relay 
I used was a 
generic 5volt relay (Radio Shack). The potentiometer should provide for 
some trim-ability on when the relay kicks on and off.  The supply voltage 
can vary, somewhat, between 5-12 volts. Just make sure the op-amps have a 
high enough voltage rating. Yes, you may substitute other op-amps if you 
choose. I used a National Semiconductor LM324, which has four amps on it, 
only two of which were needed for the filter.

Watch out to not over drive the input. Don't connect the speaker output to 
this circuit. More protection circuitry is needed if the circuit is being 
driven off of a speaker port. To give you that protection circuit, I would 
need to know exactly what this circuit is being attached to.

Good luck, and let me know how this turns out.

Abtin Spantman

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